Ion beam generating system



Nov. 3, 1964 P. FRENCH 3,155,857

ION BEAM GENERATING SYSTEM Filed June 1, 1960 masowacz 49 2a 25 24 23 22D CsouQcE /38 g INVENTOR. z Park Fiencb ATTYS.

United States Patent 3,155,857 ION BEAM GENERATING SYSTEM Park French,Aurora, Ohio, assignor to Thompson Ramo gooldridge Inc., Cleveland,Ohio, a corporation of bio Filed June 1, 1960, Ser. No. 33,332 16Claims. (Cl. 313-63) This invention involves an ion beam generatingsystem in which a highly stable neutralized ion beam is produced in asimple and reliable manner.

The invention involves the use of positively charged ions which areelectrically charged atoms or molecules, formed by the loss of one ormore electrons. Positively charged ions are attracted to a negativelycharged body in the same way that a piece of lint may be attracted to acomb. They are repelled by a positively charged body or by otherpositively charged ions. Positively charged ions may be emitted intoouter space, or into an evacuated space, by heating certain materials toan elevated temperature. Their speed may be increased by disposing inspaced relation to the emitting surface an accelerating electrode orgrid at a negative potential, so that they may be discharged into outerspace or into an evacuated space. They may thus be used for propellingspace vehicles and are also usable in other applications. However,certain problems are involved.

One problem is that the potential of the space vehicle or emitting bodyshould be maintained constant. This problem can be readily solved by theseparate emission of an electron beam with the total electronic currentflow being equal to the ion current flow.

Another problem, not so easily solved, is that of an ion cloud, createdwhen the ions are emitted in a broad beam, i.e. one in which theaccelerating grid is spaced from the ion-emitting surface a distancewhich is small in relation to the transverse dimensions of the surface.In particular, the ions after passing the accelerating grid are sloweddown by the electrostatic repelling forces exerted by other positivelycharged ions in the region. As a result, a cloud of ions starts to buildup beyond the accelerating grid. As additional ions approach the ioncloud, they are slowed down even more and may be turned back toward theemitter. Ultimately, a condition is produced in which the ion cloud actsas the virtual ion source, which may be at a distance from theaccelerating electrode approximately equal to the distance from theemitting to the accelerating electrode. The ions returned to the emitterare substantially equal in number to those emitted and there issubstantially no net current flow. As a result, eifective propulsion isnot possible.

It is possible to minimize the ion cloud problem by emitting the ions ina narrow beam, with the accelerating grid being placed at a substantialdistance from the emitting surface, in relation to the transversedimensions of the surface. Under such circumstances, the electric fieldin the ion beam acts mainly in a transverse direction so as to make thebeam gradually expand, and not substantial ion cloud may be produced.However, to obtain sufiicient ion generation in many practicalapplications, it would be necessary to operate a plurality of the narrowbeam sources in parallel side-by-side relation. And when so operated, itis found that the beams interact and that the total effect issubstantially the same as produced by the broad beam emitter. The ioncloud problem is not solved.

To solve the ion cloud problem, it has been proposed to emit electronsdirectly into the ion beam to travel along with the beam and neutralizethe charge thereof, and thus prevent formation of the cloud. It isfound, however, that it is virtually impossible to emit Patented Nov. 3,1964 electrons at the required speed. The reason for this is that thetravel of ions is quite slow in relation to the speed of electrons froma conventional emitter, ions being much, much heavier than electrons.Sufficiently slow electrons cannot be produced by any known andpractical means. In addition, even if a practical emitter could befound, it would be practically impossible to avoid violent fluctuationsin potentials and instabilities. This is due to the fact that an ioncloud would almost certainly accelerate any slow electrons that might bepresent. For example, an ion beam by itself might generate a potentialon the order of 1,600 volts in a distance equal to about of itsdiameter. Even if the beam were balanced 99% perfectly, it would stillbuild up several volts in the same distance, and electrons would beaccelerated by such potential differences. The beam would thereforecontain electrons of energy primarily determined by the potentialfluctuations within itself. Furthermore, such an arrangement has certainfeatures of inherent instability. With an ion beam in an approximatestate of balance a positive fluctuation in potential will accelerateelectrons to reduce their charge density and the increased net positivespace charge further increases the potential fluctuation. As a result,violent fluctuations in potential can occur even though the total numberof electrons and ions is the same.

This invention was evolved with the object of providing a practicalsolution to the problems discussed above. By this invention, anextremely stable neutralized ion beam is produced in a simple and yetvery reliable manner.

According to this invention, electrons are injected into an ion beam totravel in spiral paths with an axial velocity closely matching the ionvelocity. Preferably, the spiral paths are of substantially constantradii, a condition which is produced by obtaining a balance between theforce exerted by the electrostatic field of the beam, urging theelectron toward the center or principal axis of the beam, and thecentripetal force, proportional to the square of the tangential velocityof the electron. It is found that the injection velocity can berelatively high, far greater than the ion velocity, so as to makepossible the use of practical electron emitters.

In addition, it has been discovered that by use of certain features andconditions, it is possible to obtain a total electron current flow sodistributed as to obtain substantially complete charge neutralization ofthe beam as well as current neutralization. Furthermore, by using suchfeatures and conditions, the neutralization can be obtained with a highdegree of stability.

Additional features of the invention relate to the physi calconstruction and geometry of the neutralization structure. Thesefeatures and other objects and advantages of the invention will becomemore fully apparent from the following detailed description taken inconjunction with the accompanying drawing which illustrates a preferredembodiment and in which:

FIGURE 1 is a diagrammatic plan view of an ion beam system constructedaccording to the principles of this invention, with electricalconnections thereto being schematically illustrated;

FIGURE 2 is a sectional view taken substantially along line II--II ofFIGURE 1; and

FIGURE 3 is a sectional view taken substantially along line IlIIlI ofFIGURE 1.

The ion beam system of this invention, generally designated by referencenumeral 10 comprises an ion emitting surface 11, an acceleratingelectrode or grid structure 12 in spaced relation to the ion emittingsurface 11, and a neutralizing structure 13 arranged to emit electronsinto the ion beam to travel in spiral paths therein and to neua) tralizethe charge of the beam, to prevent formation of an ion cloud.

The illustrated ion emitter comprises a porous tungsten wafer 14supported on and closing the upper end of a hollow cylindrical body 15,which may preferably be of molybdenum. The body 15 has an intermediateWall 16 dividing the space therewithin into a chamber 17 below thediaphragm 14 and a cavity 18 in which a heating coil 19 is disposed. Thestructure is supported on a tube 20, the upper end of which is fittedinto a central opening of the wall 16. Tube 20 is preferably ofmolybdenum.

In operation, cesium vapor is fed upwardly through the tube 20 into thechamber 17 below the porous tungsten wafer 14. The structure is heatedby the heating coil 19 and the cesium vapor diffuses through the poresof the porous tungsten wafer 14, to result in the emission of ions fromthe surface 11, the upper surface of the wafer 14.

The ions emitted from the surface 11 are accelerated in speed by theelectric field created by the accelerating electrode or grid structure12. The structure 12 may preferably comprise a series of concentriccircular wires 21-26, disposed on suitable supports 27 of insulatingmaterial. The grid wires 21-26 may be connected together electricallyand to the negative terminal 28 of a direct current source 29 having apositive terminal 30 connected to ground.

The neutralizing structure 13 comprises a series of circular concentricgrid wires 31-36 disposed on radially extending supports 37 ofinsulating material. The grid wires 31-36 establish the generalpotential distribution in the neighborhood of the device. Electrons areinjected from a radially extending cathode 38, supported on one of thesupports 37, and having two active sides 39 and 40, from which electronsare emitted. To create an extraction field, a plurality of invertedU-shaped grid elements 41-46 are disposed over the cathode 38 and arerespectively connected to the circular grid wires 31-36. The verticalleg portions of the elements 41-46 are disposed in planes in spacedrelation to the active surfaces 39 and 40 of the cathode 38.

The relationship of the extracting grid elements 41-46 with respect tothe cathode 38 is symmetrical, so that equal currents are emitted fromboth sides 39 and 46 of the cathode 38. The current from the side 3? isused for neutralization, while the current from the side 40 isultimately collected by the circular grid structure 31-36 Where it isused in compensating circuitry to adjust the current from the side 39 tothe values required for neutralization. In particular, a barrier orinactive strip 47 is disposed along the upstream edge of the side 40 toblock emission. This creates a thin unneutralized ion layer between thecircular grids 31-36 and the electron stream from the side 40, andcauses the electron stream to be diverted in the upstream direction soas to impinge upon the circular grid structure 31-36. At the same time,the stream of electrons from the side 39 is diverted in a downstreamdirection, to cause the electrons to move in spiral paths at an axialvelocity matching the ion velocity.

An arrangement for establishing proper operating potentials isschematically illustrated in FIGURE 1. The outer end of the cathode 38is connected to a terminal 48 of a direct current source 49. The innerend of the cathode 38 is connected through a resistor 50 to anotherterminal 51 of the source 49, terminal 51 being connected to ground.Wires 31-36 are respectively connected to ground through resistors52-57.

The form and values of the various elements and components of the systemrequired to obtain optimum operatron are dictated by parameters of theion emitter such as the shape of the emitter surface 11 and thepotential distribution and charge densities produced by the surface 11and the accelerating grid structure 12. In any case, the electronsshould be emitted in such directions and at such velocities as to travelin generally spiral paths and should be so distributed as to effectivelyneutralize the ion beam through a substantial distance. With a circularion beam source as illustrated, and with uniform ion charge and currentdensities, it has been found that certain conditions should prevail toobtain optimum performance.

In particular, the neutralizing structure should fulfill the followingconditions:

1. The radial potential behavior of the physical structure should matchthat of the ion beam. At any radius r the potential should equal where ais the distance from the principal beam axis to the outer beam boundaryand -V is the potential at the outer beam boundary.

2. The electrons should be injected with their average axial velocitycomponent closely matching the ion velocity.

3. The azimuthal velocities must have the proper variation with radius.If it is assumed that the azimuthal motion carries all the injectionenergy, which is very nearly the case, the injecting cathode potentialat all radii should equal twice the beam potential.

4. Any radial velocities should be small relative to azimuthalvelocities and should average zero at any radius.

5. The injected current per unit radius should be proportional to theradius, or should equal Kr, where K is a constant.

6. The net injected electron current should equal the ion current. As apractical matter, there will be a miniminimum radius r at whichneutralization can be effected, due to limitations on the minimumpossible azimuthal electron velocites. The net electron current shouldequal To establish the proper operating potentials of the circular grids31-36, it is found that the resistance from each grid element to ground(the resistance of each resistor 52-57) should be equal to where V isthe potential at the outer beam boundary, It is the number of elementsper unit radius, r is the radius of the grid element, a is the beamradius, r is the minimum radius of the beam, and I is the net injectedcurrent which would equal the iron current. This equation assumes thegrid to be uniformly spaced.

It might here be noted that the grid wires may preferably be of no-sagtungsten to prevent high temperature creep.

With regard to the cathode circuit, the cathode potential should betwice that of the beam at all radii, as noted above. Thus the DC source49 should supply a voltage equal to twice the potential at the outerbeam boundary and the cathode 38 must have the proper resistance valuesalong its length. The resistor 50 must also have the proper value.However, it should be noted that although it is thus necessary todissipate some power along the cathode to maintain proper potentials, itis not necessary to do so for the purpose of heating, since the cathodeis in close proximity to the hot ion emitter and is heated therefrom.The temperature of the emitter may be on the order of 1200 C., forexample. For operation in the neighborhood of 1100 C., impregnatedcathodes are satisfactory, such as sintered tungsten powder impregnatedwith a mixture of aluminum oxide, and barium and calcium carbonates,which become oxides upon heating. For operation around 900 C., a nickelcathode coated with the usual barium and strontium carbonate emitter mixwould be preferable.

To obtain the proper potential at the inner end of the cathode 38, it isfound that the resistor 50 should have a resistance equal to where V isthe potential at the outer beam boundary, a is the radius at the outerbeam boundary, r is the minimum beam radius and I is the total injectedcurrent, which should be equal to the ion current.

It is found that the resistance per unit length of the cathode should beequal to This cathode resistance can be obtained with a tapered slabcathode which increases in thickness linearly with radius since forohmic resistors of slowly changing crosssection, A, the change inresistance per unit radii is equal to R /A where R is the volumeresistivity.

If it is assumed that the cathode has a height it its thickness at themaximum radius, a, is equal to 4ahV 1 R I; (1

With regard to the extracting grid geometry, it may be assumed that thecathode is run space-charge limited, which is desirable for purposes ofstability with respect to aging. It may be also assumed that the legs ofthe elements 41-46 are positioned so as to form a good approximation toa plane situated close to the emitting faces 39, 40 of the cathode 38,which is desirable to fulfill the condition of small radial electronvelocities. Under such conditions, it is found that the grid-cathodespacing necessary to obtain an injected current per unit radiusproportional to the radius and the proper total current is equal towhere e is the electron charge, m is the electron mass and c is thevelocity of light, the other parameters being as above described.

It will be observed that the grid-cathode spacing increases linearlywith the radius and since the cathode faces lie along radii, the gridplanes lie along radii also, as is illustrated.

The grid-cathode spacing as expressed above is the spacing in the casewherein no ions exist in the region between the cathode and the grid.Accordingly, the spacing may be increased somewhat from the valueexpressed or in the alternative, the values of the grid resisters may bechanged somewhat.

In the operation of the device, the voltage of the acceleration gridsource 29 may be adjusted to insure optimum operation. The voltageshould preferably be such that the upstream current from the cathodeface 49 is removed completely after one revolution, i.e. the pitchshould be adjusted to be equal to the electron beam width. Under suchcircumstances, the direction and pitch of the downstream electron streamwill automatically be established. It should be noted that the electronemission is self-regulating to a degree. If, for instance, a givenregion of the cathode gives insufficient emission, the circular grid atthat radius will intercept less current than is normal, and its voltagewill be raised because of the lowered current flowing through theassociated grid resistor. Since the extracting grid loops are attachedto circular grid elements at the same radius, the extracting voltagewill be raised at the radius of low emission, to provide a compensatingincrease in extracted current and electron space charge. The operationof this compensating mechanism depends upon symmetric properties on bothcathode faces, and care should be taken to insure that condition.

As noted above, radial velocities must be small rela: tive to azimuthalvelocities and should average zero at any radius. Care should be takento insure that this is the case. It is particularly important that therebe a sufiicient number of extracting grids at the proper potentials andin many cases it will be desirable to provide many more than the sixthat are used in the device as diagrammatically illustrated. In thisconnection, it is noted that the cathode thickness, grid spacings, etc.of the device are exaggerated in order to more clearly show theimportant details of the design.

With the system of this invention as above described a highly stable andfully neutralized ion beam is produced, permitting the attainment of alarge propulsive force while maintaining constant the potential of aspace vehicle or other emitting body. It is found that the action of theneutralization mechanism is such as to provide an automatic compensatingaction with respect to expansion of the ion beam and with respect tounavoidable random variations, so as to minimize the production ofunstable conditions.

It is important to note that although the principles of the inventionhave herein been specifically applied to a system for neutralizing acircular ion beam having uniform charge and current densities, it willbe apparent that such principles can be applied as well to theneutralization of beams of other forms, by making suitable modificationsin the design and proportions of the system.

It will be understood that other modifications and variations may beeffected without departing from the spirit and scope of the novelconcepts of this invention.

I claim as my invention:

1. In an iOn beam system, means for emitting an ion beam having aprincipal axis, electron-emitting means disposed in said beam foremitting electrons at points spaced from said axis to travel ingenerally spiral paths about said axis, said electron-emitting meanshaving a radial potential behavior closely matching that of said beam.

2. In an ion beam system, means for emitting an ion beam having aprincipal axis, electron-emitting means disposed in said beam foremitting electrons at points spaced from said axis to travel ingenerally spiral paths about said axis, said electrons being emitted atvelocities which increase with the distance from said axis.

3. In an ion beam system, means for emitting an ion beam of generallycircular cross-section having a principal axis with substantiallyuniform charge and current densities, electron-emitting means disposedin said beam for emitting electrons at points spaced from said axis totravel in generally spiral paths, said electron-emitting means having apotential at each point thereof substantially equal to where r is thedistance from said axis to said point, a is the distance from said axisto the outer beam boundary and -V is the potential at said boundary.

4. In an ion beam system, means for emitting an ion beam of generallycircular cross-section having a principal axis with substantiallyuniform charge and current densities, electron-emitting means includinga radially extending cathode disposed in said beam for emittingelectrons at points spaced from said axis to travel in generally spiralpaths, said cathode having an injection potential at all radiisubstantially equal to twice the beam potential.

5. In an ion beam system, means for emitting an ion beam of generallycircular cross-section having a principal axis with substantiallyuniform charge and current densities, electron-emitting means includinga radially extending cathode disposed in said beam for emittingelectrons at points spaced from said axis to travel in generally spiralpaths, the injected electron current per unit radius being proportionalto the radius.

d 6. In an ion beam system, means for emitting an ion beam having aprincipal axis, a cathode extending radially outwardly with respect tosaid axis and having an eleetron-emittingsurface in a radial plane toemit electrons to travel in spiral paths about said axis.

7. In an ion beam system, means for emitting an ion beam having aprincipal axis, a cathode extending radially outwardly with respect tosaid axis and having an electronemitting surface in a radial plane toemit electrons to travel in spiral paths about said axis, and a gridstructure disposed in spaced relation to said electron-emitting servicefor creating an extraction field.

8. In an ion beam system, means for emitting an ion beam having aprincipal axis, a cathode disposed radially in said beam for emittingelectrons to travel in generally spiral paths about said axis, and apotential-fixing grid structure associated with said cathode anddisposed in said beam in a plane transverse to said axis.

- 9. In an ion beam system, means for emitting an ion beam having aprincipal axis, a cathode disposed radially in said beam for emittingelectrons to travel in generally spiral paths about said axis, and apotential-fixing grid structure associated with said cathode anddisposed in said beam in a plane transverse to said axis, said gridstructure including a plurality of generally circular elements havingcenters coincident with said axis.

10. In an ion beam system, means for emitting an ion beam having aprincipal axis, a cathode extending radially outwardly with respect tosaid axis and having an electronemitting surface in a radial plane toemit electrons to travel in spiral paths about said axis, apotential-fixing grid structure associated with said cathode anddisposed in said beam in a plane tranverse to said axis, and anextraction grid structure connected to said potential-fixing gridstructure and disposed in spaced relation to said electron-emittingsurface for creating an extraction field.

11. In an ion beam system, means for emitting an ion beam having aprincipal axis, a cathode extending radially outwardly with respect tosaid axis and having an electronemitting surface in a radial plane toemit electrons to travel in spiral paths about said axis, apotential-fixing grid structure associated with said cathode anddisposed in said beam in a plane transverse to said axis, saidpotentialfixing grid structure including a plurality of generallycircular elements having centers coincident with said axis, and anextraction grid structure including a plurality of elements connected tosaid circular elements and disposed in spaced relation to saidelectron-emitting surface for creating an extraction field.

12. In an ion beam system, means for emitting an ion beam having aprincipal axis, a cathode extending radially outwardly with respect tosaid axis and having an electron-emitting surface in a radial plane toemit electrons to travel about said axis, and means for establishing afield active on the emitted electrons to cause said electrons to travelin spiral paths at a speed closely matching the ion speed.

13. In an ion beam system, means for emitting an ion beam having aprincipal axis, a cathode extending radially outwardly with respect tosaid axis and having on opposite sides thereof a pair ofelectron-emitting surfaces disposed in radial planes, a grid structureassociated with said cathode for fixing the potential in the regionthereof and to extract electrons from said surfaces to cause electronflow from said surfaces in two oppositely oriented streams about saidaxis, and means for causing one of said streams to spiral upstream to becollected by said grid structure and to cause the other of said streamsto spiral downstream to neutralize the ion beam.

14. In an ion beam system, means for emitting an ion beam having aprincipal axis, a cathode extending radially outwardly with respect tosaid axis and having on opposite sides thereof a pair ofelectron-emitting surfaces disposed in radial planes, a grid structureassociated With said cathode for fixing the potential in the regionthereof and to extract electrons from said surfaces to cause electronflow from said surfaces in two oppositely oriented streams about saidaxis, and a barrier strip along the upstream side of one of saidsurfaces to cause one of said streams to spiral upstream to be collectedby said grid structure and to cause the other of said streams to spiraldownstream to neutralize the ion beam.

15. In an ion beam system, means for emitting a beam of positivelycharged ions having a principal axis wherein the ions travel generallyrectilinearly in paths parallel to said principal axis, and means foremitting negatively charged electrons into said beam at points spacedfrom said principal axis and in directions such as to cause theelectrons to travel spirally about said principal axis.

16. In an ion beam system, means for emitting a beam of positivelycharged ions having a principal axis wherein the ions travel generallyrectilinearly in paths parallel to said principal axis, and means foremitting negatively charged electrons into said beam at points spacedfrom said principal axis and in directions such as to cause theelectrons to travel spirally about said principal axis, the axialvelocity of said electrons being closely matched to the velocity of saidions.

References Cited in the file of this patent UNITED STATES PATENTSLindenblad Oct. 9, 1956 Bell et al Aug. 22, 1961 OTHER REFERENCES

1. IN AN ION BEAM SYSTEM, MEANS FOR EMITTING AN ION BEAM HAVING APRINCIPAL AXIS, ELECTRON-EMITTING MEANS DISPOSED IN SAID BEAM FOREMITTING ELECTRONS AT POINTS SPACED FROM SAID AXIS TO TRAVEL INGENERALLY SPIRAL PATHS ABOUT SAID AXIS, SAID ELECTRON-EMITTING MEANSHAVING A